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Metrology: Hand and Eye

Laser scanning has come a long way over the last few years. Fueled by advances in information technology and the continuing march toward Industry 4.0, the devices have become more accurate and more affordable, capable of gathering millions of data points much more quickly and easily. The process has become steadily more intuitive, requiring minimal training, and the software has continued to enable more functionality and improve existing processes.

Compared with contact metrology, laser scanning is undoubtedly faster, more portable and capable of gathering a much greater volume of data points in a single pass. But there are unique use cases for each kind of metrology.

A 2016 article on Engineering.com referred to a poll that found users of laser scanning technology valued it for accuracy, speed and portability. Other advantages include colour capture/high definition resolution, cost and ease of use.

Click image to enlargeThe article’s headline provocatively suggested that laser scanning is simply better than contact metrology, but while the article did tout the advantages of optical scanning, it also made the point that optical and contact metrology each have their advantages.

Any choice between options entails a compromise, and with laser scanning the big compromise is–and will continue to be for the foreseeable future–accuracy of measurement. Optical methods will continue to be most appropriate where the highest possible degree of precision isn’t required. But because line of sight is required, they are susceptible to poor environmental conditions and are more difficult to bring to bear on deeply recessed features or oblique surfaces. The degree of surface reflectivity can also create problems.

However the increased data point density of laser scanning is crucial when inspecting a free form surface, says Peter Detmers, president of Mitutoyo Canada. He uses the example of a car body.

“If you want to make sure the front doors are flush with the rear ones, or the truck is flush to the fenders, you don’t have big issues with those surfaces. Gathering points data around locations where those gaps would be is highly important in order to verify those values. You want high speed and the high data point collection, but relatively low accuracy is required.”

For higher degrees of accuracy, Detmers says contact measurement or a traditional CMM will still be the preferable option.

“If you look at an example like an engine block, there’s nothing, aside from the casting, on an engine block that would be conducive to that type of laser or optical measurement,” Detmers says. “What’s important is that cylinder bore. You can only ‘see’ the surface, you can’t see 50 mm down from the front face, and what you can’t see, you can’t measure.

Click image to enlarge“Once you machine something, you’re starting to get out of that realm of optical scanning technology and more into tactile type probing, or a three-dimensional vision system where you can employ either type of measurement–optical and tactile.” Mitutoyo’s Quick Vision is one such platform.

The Engineering.com article also noted that laser and contact metrology can be useful in tandem. In one example taken from the aerospace industry–which is notorious for demanding a high degree of precision–a company acquired laser scanners to reduce the workload on its CMM. It still uses the CMM for post machining inspection, but it puts the lasers to work scanning the part before machining begins to ensure there’s enough material to be machined.

“That’s a great application for [laser scanning], and it’s quick,” says Jamie King, regional manager, Canada for Blum-Novotest. “They don’t want to find out six hours into machining that one part of the casting didn’t have enough material on it to actually machine a feature. So they scan first. You don’t need micron accuracy; you just need to make sure there’s material.” SMTLaser scanning has come a long way over the last few years. Fueled by advances in information technology and the continuing march toward Industry 4.0, the devices have become more accurate and more affordable, capable of gathering millions of data points much more quickly and easily. The process has become steadily more intuitive, requiring minimal training, and the software has continued to enable more functionality and improve existing processes.

Compared with contact metrology, laser scanning is undoubtedly faster, more portable and capable of gathering a much greater volume of data points in a single pass. But there are unique use cases for each kind of metrology.

A 2016 article on Engineering.com referred to a poll that found users of laser scanning technology valued it for accuracy, speed and portability. Other advantages include colour capture/high definition resolution, cost and ease of use.

The article’s headline provocatively suggested that laser scanning is simply better than contact metrology, but while the article did tout the advantages of optical scanning, it also made the point that optical and contact metrology each have their advantages.

Click image to enlargeAny choice between options entails a compromise, and with laser scanning the big compromise is–and will continue to be for the foreseeable future–accuracy of measurement. Optical methods will continue to be most appropriate where the highest possible degree of precision isn’t required. But because line of sight is required, they are susceptible to poor environmental conditions and are more difficult to bring to bear on deeply recessed features or oblique surfaces. The degree of surface reflectivity can also create problems.

However the increased data point density of laser scanning is crucial when inspecting a free form surface, says Peter Detmers, president of Mitutoyo Canada. He uses the example of a car body.

“If you want to make sure the front doors are flush with the rear ones, or the truck is flush to the fenders, you don’t have big issues with those surfaces. Gathering points data around locations where those gaps would be is highly important in order to verify those values. You want high speed and the high data point collection, but relatively low accuracy is required.”

For higher degrees of accuracy, Detmers says contact measurement or a traditional CMM will still be the preferable option.

“If you look at an example like an engine block, there’s nothing, aside from the casting, on an engine block that would be conducive to that type of laser or optical measurement,” Detmers says. “What’s important is that cylinder bore. You can only ‘see’ the surface, you can’t see 50 mm down from the front face, and what you can’t see, you can’t measure.

“Once you machine something, you’re starting to get out of that realm of optical scanning technology and more into tactile type probing, or a three-dimensional vision system where you can employ either type of measurement–optical and tactile.” Mitutoyo’s Quick Vision is one such platform.

The Engineering.com article also noted that laser and contact metrology can be useful in tandem. In one example taken from the aerospace industry–which is notorious for demanding a high degree of precision–a company acquired laser scanners to reduce the workload on its CMM. It still uses the CMM for post machining inspection, but it puts the lasers to work scanning the part before machining begins to ensure there’s enough material to be machined.

“That’s a great application for [laser scanning], and it’s quick,” says Jamie King, regional manager, Canada for Blum-Novotest. “They don’t want to find out six hours into machining that one part of the casting didn’t have enough material on it to actually machine a feature. So they scan first. You don’t need micron accuracy; you just need to make sure there’s material.” SMT